output: use Hash.new {|hash, key| ... } for default value

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
This will remove evaluating meta_and_data[meta] ||= [] in the iterator every time to improves performance slightly.

The results of actually reading a 10 GB file are as follows.

• Before: 67.50262675 sec
• After: 63.862033033 sec

Here is micro benchmark code.

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
end

Benchmark.ips do |x|
  h1 = {}
  h2 = Hash.new { |h, k| h[k] = [] }
  key = :foo

  x.report("1") {
    h1[key] ||= []
    h1[key][0] = 1
  }

  x.report("2") {
    h2[key][0] = 1
  }

  x.compare!
end

ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
Warming up --------------------------------------
                   1     1.776M i/100ms
                   2     2.239M i/100ms
Calculating -------------------------------------
                   1     18.426M (± 1.5%) i/s   (54.27 ns/i) -     92.366M in   5.013927s
                   2     23.545M (± 0.8%) i/s   (42.47 ns/i) -    118.653M in   5.039795s

Comparison:
                   2: 23544712.1 i/s
                   1: 18426072.0 i/s - 1.28x  slower

Docs Changes:

Release Note: